Answer:
Hablan
Step-by-step explanation:
My friends were on the phone last night with the teacher.
The infinitive 'hablar' means 'to talk' or 'to speak'.
Conjugations
yo - hablo
tu - hablas
el/ella/ud. - habla
nostros - hablamos
vosotros - hablais
ellos/ellas/ustedes - hablan
The pronoun 'mis amigos' or 'my friends' is they which refers to they (masculine) or (feminine) then you (plural) in formal tone.
This would mean the form would be hablan.